The present invention relates to an information reproducing apparatus for reproducing information stored in a recording medium (e.g., an optical disk) which has an information storage track, and more particularly to a device for detecting the crosstalk level of a signal which is optically read from the recording medium.
Optical disks such as laser vision disks or the like have information storage tracks for recording information in the form of signal pits arranged as spiral turns extending progressively from the inner circumferential edge toward the outer circumferential edge of the optical disks. The optical disks with such information storage tracks, however, suffer the problem of crosstalk in which a signal read from a track which is being presently traced or read is affected by information recorded in an adjacent track. One of the causes of the crosstalk problem is that a laser beam spot applied to the optical disk is so large as compared with the distance between two adjacent tracks that when the laser beam spot is falling on one track, it is also applied to an adjacent track. Therefore, the laser beam also reads the information recorded in the adjacent track as well as the recorded information from the track which is being presently traced.
There are known two solutions to the crosstalk problem of the information read from optical disks. According to one of the solutions, any inclination of an optical disk being read with respect to an optical pickup is detected, and the optical pickup is corrected in its attitude so that it correctly faces the information recording surface of the optical disk. More specifically, light emitted from a light source such as a light-emitting diode or the like is applied to the optical disk, and light reflected from the optical disk is detected by two photodetectors which are positioned on each side of the light source. The levels of output signals from the photodetectors are compared with each other by a differential amplifier to detect any inclination of the optical disk. Depending on the detected disk angle, the light source and the photodetectors are positionally corrected into correctly facing relationship to the optical disk through a feedback loop. However, this arrangement has certain drawbacks. The pickup is necessarily large in size. At an outer circumferential region of the optical disk, a portion of the light emitted from the light source may not be applied to the optical disk, and hence the photodetectors may not produce output signals of proper values.
According to the other solution, three adjacent tracks on an optical disk are simultaneously read, and the detected signals are subjected to a subtraction process. One typical correcting process is disclosed in Japanese Laid-Open Patent Publication No. 57-5824. In the disclosed process, three adjacent tracks are simultaneously read, and the read signal values are subtracted to remove the crosstalk effect. With this process, however, the pickup is complex in structure. Since the signals read from the adjacent tracks also contain crosstalk from the next tracks adjacent to the adjacent tracks, the recording density of the optical disk used with this process should be limited in the range of making such crosstalk negligible.